The maintenance of a hydroelectric dam requires the inspection and repair of structures that may be submerged. A simple visual inspection from images or videos may provide qualitative information. This type of inspection allows for example verifying the surface state of the structures or detecting the presence of cracks. However, to establish a more workable evaluation of the state of the installations, it is necessary to have quantitative information. Such information allows, for example, not only detecting cracks but also accurately locating and gauging them, or establishing the complete survey of a structure to estimate its deformation.
The inspection of an underwater structure may be achieved in different ways. A first solution consists in sending divers to take spatial measurements of the structure. The execution of the inspection is then subjected to several constraints. The area must first be physically accessible and this access must be authorized by the safety rules. The environment of a dam indeed piles up the dangers of the underwater environment and the industrial environment. The divers then need enough visibility to be able to take the measurements. The water may be clouded by particles in suspension coming from the vegetation, aquatic organisms or the corrosion of metallic pieces. Furthermore, the quality of the measurements depends on the skill and the experience of the divers. Since the taking of the measurement is manual, the results comprise no notion of confidence or uncertainty. Finally, the time needed for the taking of a measurement may be long whereas the stopping of a part of a dam during the inspection may be very expensive for the operator. All of these constraints result in that the installations that require it are not always regularly inspected. In that case, the state of the dam deteriorates until the day one of its parts malfunctions. To sum up, an inspection with divers is not always possible, has a high cost, exhibits dangers for the human life and limits a rigorous quantitative use of the results.
A second solution for the underwater inspection resides in the use of a robotic system. In general, it may be any remote-controlled or autonomous system allowing collecting the data required for the inspection. In that class are found the mobile robots such as the ROV (“Remotely Operated Vehicle”) and the AUV (“Autonomous Underwater Vehicle”). The use of a robot provides numerous advantages and new capabilities with respect to an inspection with divers. The safety requirements are not the same since no operator is directly present in the area to be inspected. The data may be collected by a larger number of various sensors: camera, video camera but also sonar, passive or active vision system. It is possible to combine the use of these sensors to take the measurements in different conditions. For example, the use of a vision system provides a good accuracy for a local inspection but an acoustic system allows collecting data over a larger range and in bad visibility conditions. The filtering and merging of the measurements coming from several sensors may reduce their uncertainty. The amount of collected data is greater. By having a sufficient measurement density, the risks of not detecting an irregularity are lower. A greater acquisition capacity makes large scale structure inspection also possible. It is for these reasons that the underwater robots are actively developed since over thirty years. The maturity of the technology now makes their use possible in the industry.
The robotized inspection systems comprise numerous advantages but their tuning poses new problems and their functioning may fail in certain specific conditions.
Although the data can be collected with different types of sensors, each sensor is efficient for a particular purpose.
One of the common problems of the underwater robots is the accurate positional tracking of the system. Yet, to be usable, the collected data need to be referenced in a same global reference system. However, on the outskirts of a dam, a robot sometimes operates in a closed environment, which may bring its position tracking system to a fault. For example, in the case of acoustic systems, the emitted waves bounce back on the walls and adversely affect the interpretation of the reflected signal. Or in the case of compasses, the earth magnetic field is distorted by the presence of the reinforcements of the concrete and prevents a good reading of the orientation.
The inspection of certain structures requires high measurement accuracy. Therefore, it is both necessary that the sensor that effectively performs the measurement be accurate and that the position tracking system of the robot be also accurate. Currently, a data measurement accuracy expressed in a global reference system in the order of the centimeter is generally reached, which is inadequate for certain applications.
It should be noted that a high measurement accuracy is not enough. A confidence level in the measurement is also required. This confidence level indicates the reliability of the system and depends of both the position tracking system and the sensor that performs the measurement.
An inspection system intended for on-site operation is subjected to perturbations commonly found in the environment (particles in suspension, vibrations, impacts). Thus, even if the system has a good measurement accuracy in ideal conditions, this accuracy will degrade in real conditions.